A boosted engine may offer greater fuel efficiency and lower emissions than a naturally aspirated engine of similar power. During transient conditions, however, the power, fuel efficiency, and emissions-control performance of a boosted engine may suffer. Such transient conditions may include rapidly increasing or decreasing engine load, engine speed, or mass air flow. For example, when the engine load increases rapidly, a turbocharger compressor may require increased torque to deliver an increased air flow. Such torque may not be available, however, if the turbine that drives the compressor is not fully spun up. As a result, an undesirable power lag may occur before the intake air flow builds to the required level.
It has been recognized previously that a turbocharged engine system may be adapted to store compressed air and to use the stored, compressed air to supplement the air charge from the turbocharger compressor. For example, Pursifull et al. describe a system in US 2011/0132335 wherein compressed air is stored in a boost reservoir and is dispensed when insufficient compressed air is available from the turbocharger compressor. By dispensing extra compressed air from the boost reservoir, torque corresponding to the dispensed air can be provided to meet the torque demand while the turbine spins up.
However, the inventors herein have identified potential issues with such a system. As one example, turbo-lag may not be sufficiently addressed even after using the dispensed air to generate torque due to low exhaust temperatures that delay the spin-up of the turbine. For example, if the boost level at the time of the tip-in is higher than a threshold, the torque compensation via the dispensed air may be sufficient to address the turbo lag. However, if the boost level at the time of the tip-in is lower than the threshold, the turbine speed may be low, and the torque compensation via the dispensed air may not be sufficient to address the turbo lag. As another example, if the boost reservoir has a small volume, the boost air may be used up relatively fast, in particular at high engine speeds, and there may not enough time to address the turbo lag. As such, engine performance may be degraded.
Thus, at least some of the above issues may be addressed by a method for a turbocharged engine comprising, in response to a tip-in, raising exhaust temperature by discharging pressurized air from a boost reservoir to an intake manifold while retarding spark ignition timing. In this way, turbine speed can be rapidly raised.
For example, in response to a tip in, an engine controller may raise the exhaust temperature by discharging an amount of pressurized air from a boost reservoir outside of a valve overlap period (e.g., during an intake or compression stroke) while retarding spark by an amount based on the discharged amount of pressurized air. As a result, the air-fuel mixture may combust in the cylinder, leading to elevated exhaust gas temperatures and expediting turbine spin-up. The amount of spark retard used may be limited such that a net engine combustion torque is maintained or increased. Consequently, turbo lag may be reduced while at least some torque compensation is provided. Overall, engine performance is improved.
It will be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description, which follows. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined by the claims that follow the detailed description. Further, the claimed subject matter is not limited to implementations that solve any disadvantages noted herein.